AG041 problems

My last hope....
We have a bulldog broadband 2mbps service
a Linksys AG041 modem router running DHCP
2 switches connected to the PCs, 8 in total, but generally only 5 connected at any one time.
Service is fine when I use it on my own, as soon as more than 2 people connect to it the whole network grinds to a halt to less than 4kb/s on the net.
Msn Messenger is a major problem to, even though UPnP is enabled in the modem interface, msn signs out and in every 10 seconds and it does this for everyone on the network.
No firewall is enabled on the router so it rules that out, ive tried resetting to factory defaults, and opening loads of different ports but nothing makes a difference, ive been in touch with Linksys and Bulldog to no avail yet.
Any help much appreciated.
thanks